Merged two validation clauses

Author: Christoph Büscher

src/main/java/org/elasticsearch/index/query/TermQueryBuilder.java

@@ -216,11 +216,8 @@ public Query toQuery(QueryParseContext parseContext) {
     @Override
     public ActionRequestValidationException validate() {
         ActionRequestValidationException validationException = null;
-        if (this.fieldName == null) {
-            validationException = ValidateActions.addValidationError("field name cannot be null.", validationException);
-        }
-        if (this.fieldName != null && this.fieldName.isEmpty()) {
-            validationException = ValidateActions.addValidationError("field name cannot be empty.", validationException);
+        if (this.fieldName == null || this.fieldName.isEmpty()) {
+            validationException = ValidateActions.addValidationError("field name cannot be null or empty.", validationException);
         }
         if (this.value == null) {
             validationException = ValidateActions.addValidationError("value cannot be null.", validationException);